Li-based thin-film secondary battery is used for various devices such as a thin film solar cell, a thin film thermoelectric element, and a wireless charging element, and its demand is growing rapidly. The Li-based thin film secondary battery is typically comprises a positive electrode composed of a LiCoO2-containing thin film containing Li and Co that is a transition metal, a solid electrolyte containing Li, and a negative electrode composed of a Li metal thin film.
For the formation of the LiCoO2-containing thin film, a sputtering method is preferably used in which a sputtering target (hereinafter abbreviated also to a target) substantially composed of the same raw material as a film to be sputtered. The sputtering method is advantageous in that conditions for the film formation are easily adjusted, and that the film can easily be formed on a semiconductor substrate.
However, when a film is formed by sputtering, there is a case in that abnormal discharge (arcing), or a discharge mark based on arc discharge is generated. Thus, a stable discharge cannot be secured, causing problems such that the target is cracked and that nodules are generated during the sputtering.
Patent Document 1 pointed out that compound oxides such as LiCoO2 have relatively low conductivity as a specific issue of thin films containing LiCoO2. When the thin films are formed by DC (direct current) discharge sputtering at a high deposition rate, micro-arc discharges are frequently generated due to charging of the target. The deposited films are damaged, accordingly. It is thus difficult to obtain a positive electrode having a large discharge capacity. Radio frequency (RF) discharge sputtering, on the other hand, has a problem of low deposition rate.
The Patent Document 1 thus discloses a method for producing a thin film secondary battery by depositing a layer containing LiCoO2 on a substrate by generating a plasma between a pair of targets that are disposed opposite to each other for the purpose of improving the deposition rate.